Electro-pneumatic control systems are increasingly being employed with process control devices, such as valve actuators and piston actuators, in order to provide better or more optimal control of fluid within a process plant. Some such electro-pneumatic control systems include one or more accessories for controlling valve and piston actuators such as volume boosters and quick exhaust valves (QEVs). A volume booster, which is typically coupled to a pneumatic actuator for a valve, increases the rate of air supplied to the pneumatic actuator, or increases the rate of air exhausted from the pneumatic actuator. This increased air movement amplifies the actuator stroke speed, thereby increasing the speed at which the actuator is able to stroke the valve plug toward its open or closed position, and thus enables the valve to respond more quickly to process fluctuations. Similar to volume boosters, QEVs increase the speed at which an actuator is able to stroke a valve toward an open or closed position.
Currently, volume boosters are utilized with pneumatic actuators in a manner that makes the actuators move very slowly in response to very small set point or control signal changes. In particular, to help stabilize overall loop dynamics, some volume boosters are designed with a built-in dead band that prevents the volume booster from becoming active in response to small amplitude change control signals. While some volume boosters have small dead bands, these volume boosters still move very slowly in response to small amplitude signal changes, becoming fast only in response to larger amplitude input signals. To adjust the sensitivity of the booster and allow actuators to respond to small amplitude signals, bypass restrictions in the form of needle valves are often incorporated in the design of volume boosters.
Positioners use travel cutoffs to ensure that full seat load is reached when a reference signal falls below a predefined threshold. When cutoffs are active, the servo is bypassed and the drive signal to the current-to-pressure (I/P) transducer is set to 0% or 100%, depending on the fail state of the given actuator and active cutoff. In order to move the valve off the seat (or off the upper travel stop), back to an active region, the valve positioner must wind or unwind the actuator pneumatics from the cutoff state.
When the valve is on the valve seat, and a small amplitude command is sent to the valve, such as a slow ramp or a small step change, servo error signals are small and there can be a significant delay before the valve responds. The delay for the valve to move off the valve seat and reach the set point can be particularly significant for large volume actuators equipped with volume boosters, because the volume boosters may not become active in response to such small signal changes. Volume boosters tend to have about 5% dead band, and rarely activate in response to signals below 5%. If the volume boosters do not activate, the positioner will fill or exhaust air from the actuator through the booster bypass restriction, thereby causing a further delay in the response of the valve.
For compressor control systems, the delay in valve response is particularly problematic, because the control logic of a compressor often requires the antisurge valve to lift off the seat quickly in response to small amplitude signals. For example, when the flow rate in a compressor system drops, it is desirable to recirculate flow around the compressor to prevent the compressor from surging. Because volumes downstream of the compressor tend to be large, flow rates generally drop gradually. If the flow rate or equivalent control variable falls below a given threshold, the compressor controller will begin to move the valve off the seat slowly to obtain the required flow rate through the compressor. To maximize compressor efficiency and prevent significant upsets in the system, it is often desirable to operate the antisurge valve in this manner as long as possible before sending a trip signal to the valve that will open it up completely.